Authigenic Pb isotopes from the Laurentian Fan: Changes in chemical weathering and patterns of North American freshwater runoff during the last deglaciation
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Climate-dependent chemical weathering trends have a strong impact on the dissolved Pb isotopic composition of continental runoff during glacial terminations, so that this tracer can be used to reconstruct the impact of the North American deglaciation on aspects of freshwater runoff and ocean chemistry. Here we present authigenic Fe-Mn oxyhydroxide-derived Pb isotope records from lower Laurentian Fan sediments that trace the local deglacial continental runoff signal through the Gulf of St. Lawrence. We use these records to investigate changes in the Pb isotopic composition of the North American runoff, and their relationship to deglacial processes. The new Pb isotope records are very similar to those from the subtropical deep NW Atlantic locations, though the new data are at higher resolution and exhibit much greater amplitudes of change due to their Laurentide Ice Sheet-proximal setting. Unradiogenic compositions (Pb-206/Pb-204 as low as 18.8 at 18.5 ka) during the latest glacial change towards highly radiogenic compositions during the early Holocene (Pb-206/Pb-204 = 20.1 at 11.2 ka). Late Holocene Pb-206/Pb-204 values return to intermediate values around Pb-206/Pb-204 = 19.6. These new data support previous inferences that the Pb isotopic composition of the deep Atlantic is controlled during deglaciation by changes in the pattern and intensity of continental chemical weathering. In more detail, the most significant change in the entire Pb isotope record starts at 12.5 ka and lasts until about 11 ka. This observation, of strong Pb isotopic changes across a climatically cold period in the North Atlantic region, suggests that at this location Pb isotopic compositions recorded in deep marine authigenic Fe-Mn oxyhydroxides not only trace deglacial changes in chemical weathering intensity but also highly depend on major freshwater runoff routes in interior North America. Our data suggest a gradual opening of the eastward freshwater runoff route in the course of the Younger Dryas. (C) 2010 Elsevier B.V. All rights reserved.